What is Cosmology? The Study of the Universe: its structure, origin, evolution, and destiny Our universal world view Our cosmological model
Cosmology through h the ages Universe models formed in many cultures
Our View of the Cosmos - the story of scientific models Astronomy has seen 3 scientific revolutions in cosmology 2nd Century: Claudius Ptolemy (Physics of Aristotle) Model: Earth-centered Cosmology Big Idea: Different laws for Earth and the cosmos 16th Century: Nicolaus Copernicus (Physics of Newton) Model: Sun-centered Cosmology Big Idea: Universal physics; same laws everywhere 20th Century: Edwin Hubble (Physics of Einstein) Model: Big Bang Cosmology Big Idea: Universe is changing, g, evolving
Earth-centered Cosmology: Claudius Ptolemy, 100-170 170 AD the natural motion of the Earth.is towards the center of the universe; that t is the reason it is now lying at the center. Aristotle, On the Heavens
Testing the Earth-centered model Prediction: Future planetary positions Observation: retrograde motion of planets Refine: epicycles Success! For 1500 years
Testing the Earth-centered model Prediction: Phases of Venus Observation: Full set of phases Crisis! i
Sun-centered Cosmology: Nicolaus Copernicus 1473-1543 1543 At rest, however, in the middle of everything is the Sun. Nicholaus Copernicus, de Revolutionibus
Testing the Sun-centered model Prediction: Future planetary positions Observation: No better than Ptolemy Refine: elliptical orbits (Johannes Kepler 1571-1630)
Testing the Sun-centered model Prediction: Observed shift in position of stars (parallax) as the earth Moves around the Sun. Observation: No shift. Crisis? No, but we had to wait until 1838 (Friedrich Bessel)
Testing the Sun-centered model Prediction: Sun at center of Cosmos Observation: Sun is not at center of universe (1918), Shapley, Leavitt Observation: The galaxy is not the entire universe (1923), Hubble, Crisis!
The discovery of galaxies 1755 Immanuel Kant: nebulae are independent systems made of stars 1771 Messier s Catalogue «Catalogue des Nébuleuses et des amas d'étoiles, que l'on découvre parmi les Étoiles fixes sur l'horizon de Paris» M82, M31 (Andromeda), M33,.. 1786 William Herschel s Catalogue of Nebulae and Clusters of Stars to be later expanded into the General Catalogue of Nebulae and Clusters of Stars (GC) by John Herschel. The CN and GC are the precursors to John Louis Emil Dreyer's New General Catalogue (NGC): NGC4151, NGC5548, 1868 William Huggings M31 spectrum: flat unlike others (planetary nebule) 1885 William Parson: spiral structures in M33, M51, M101 1908 Henrietta Swan Leavitt s L-P cepheid correlation in Annals of the Astronomical Observatory of Harvard College 1917: Herber Curtis nobas in M31 hence M31 at great distance 1920: Sharpley-Curtis Great Debate -> start of Extragactic Astronomy 1925-29: Edwin Hubble identifies cepheids in M31, M33 and IC1613 -> precision distances Cepheids in M33 (Hubble, ApJ, 63, 236, 1926)
The birth of Cosmological Physics 1917 Albert Einstein: General Relativity Developments by Willem de Sitter, Karl Schwarzchild, Arthur Eddington and 1922 Alexander Friedman s expanding universe solution 1927 George Lemaitre s Primival Atom, independent of Friedman s calculations 1930 Fred Hoyle in a BBC program coined the Big Bang term
Olbers Paradox Named for Wilhelm Olbers (1758-1840), but known to Kepler and Halley Consider spherical shell of radius r and thickness dr Number of stars in this shell is 4πr 2 n dr, where n is number density of stars Light from each star is L/4πr 2, therefore light from shell is nl dr, independent of r therefore, in infinite universe, night sky should be infinitely bright (or at least as bright as typical stellar surface stars themselves block light from behind them) Why is the sky dark at night? 13
Olbers Paradox: solution(s) Light is absorbed by intervening dust suggested by Olbers doesn t work: dust will heat up over time until it reaches the same temperature as the stars that illuminate it Universe has finite size suggested by Kepler this works (integral is truncated at finite r) but now Newtonian universe will definitely collapse Universe has finite age equivalent to finite size if speed of light finite light from stars more than ct distant has not had time to reach us (currently accepted explanation) Universe is expanding effective temperature t of distant starlight is redshifted down this effect not known until 19 th century (does work, but does not dominate (for stars) in current models) Olbers + Newton could have led to prediction of expanding universe
Changing g Worldviews Age Universe 100 years AD --- The Earth + Celestial Sphere 400 years ago --- The Solar System 100 years ago --- The Milky Way 75 years ago --- The Modern Universe (2 Gly in *radius*) Today --- An Infinite Universe (the visible part has a radius of ~45 Gly)
Credits Colliding galaxies: NASA & the Hubble Heritage Team (STScI) Phases of Venus: Albert Van Helden Star field: NASA/GSFC Andromeda: Palomar Observatory, P. Challis, CfA HGC 87: Gemini Observatory/GMOS-S Galaxy cluster: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT Orion nebula: MicroObservatory, SAO Galaxy data: courtesy Emilio Falco, CfA Comic microwave background: NASA/WMAP Sun: SOHO/NASA/ESA Cecilia Payne: President and Fellows of Harvard College Early galaxies: NASA, Richard Griffiths/JHU, Medium Deep Survey Team Edge-on galaxy: Bruce Hugo and Leslie Gaul, Adam Block (KPNO Visitor Program), NOAO, AURA, NSF Ring nebula: H. Bond et al., Hubble Heritage Team (STScI / AURA), NASA White dwarf accretion: STScI, NASA Supernova Remnant: NASA/ESA/JHU/R.Sankrit & W.Blair Galaxy and supernova data: High-Z Supernova Search Team, HST, NASA Composition of the universe: SAO Spacecraft and Einstein probes: NASA ALBERT EINSTEIN and related rights / of The Hebrew University of Jerusalem, used under license. Represented by the Roger Richman Agency, Inc., www.albert-einstein.net Please contact einstein2005@cfa.harvard.edu for more information about non-credited historical images.